Is there really strength in diversity as the U.S. claims? Plato, for one, disagreed. Roman generals always used the diversity of people they fought against as a means to "divide and rule". They used these tactics against Britain since Britain around 100 B.C. was a tribal nation made up of groups of Celts and Germans. They were too divided to resist the unified Romans which is something Caesar understood very well.
The Greeks always analysed disunity in other societies. They understood if you arm a dozen or so ethnic groups within a country, the war is half-won before you even send your troops in.
The U.S. in my opinion never was a superpower in the sense it's been portrayed. It was the same thing with the USSR. Both countries were propped up only by nuclear weapons but internally there were all sorts of problems. The U.S. has never been tested by an invading army as Russia was in WW2 and always relied on a huge stack of weapons controlled by a political elite at the top. However, beneath all that, the U.S. has many many chinks in its armour.